Rare earth elements are also referred to as Rare Earth and refer to a group consisting of 17 elements of scandium (Sc/21), yttrium (Y/39), and lanthanum (La/57) to lutetium (Lu/71) (the terms inside the parentheses represent the symbol of element/atomic number). In the position of the periodic table, rare earth elements are elements from the 4th period to the 6th period in group 3. Hereinafter, an element may be represented by the symbol of element.
Rare earth elements have similar chemical properties with each other. The elements other than scandium which has a little different properties and promethium which is not present in nature occur together in the same ore and are hardly separated as an element. On the other hand, although rare earth elements are present in the earth's crust at a higher proportion than gold, silver, and the like, rare earth elements are classified into rare metals since it is difficult to separate and purify the single element.
The rare earth elements have wide applications for permanent magnets, catalysts, phosphors, and the like and are indispensable in the most advanced industries. However, with respect to the supply structure of rare earth elements, the production of rare earth elements are unevenly distributed across the countries, and vulnerability is pointed out. For stably securing future rare earth resources, recycling (recovery) will be important in addition to a search for new ore deposits, development of alternate materials, reduction in the amount of use, and a strategic stockpile.
Known methods for recovering a rare earth element dissolved in an aqueous solution include a solvent extraction method utilizing a phosphate-based extractant (Patent Literature 1) and a precipitation method with an alkali, oxalic acid, and the like (Patent Literature 2).
A solvent extraction method such as that described in Patent Literature 1 is a method capable of performing continuous operation and excellent in separation performance, but the method requires large-scale facilities and is not efficient when the concentration of an objective material to be recovered is low. A precipitation method such as that described in Patent Literature 2 is a method that is simple and excellent in cost, but the method is not suitable as a separation technique from a dilute solution.
When the concentration of an element to be separated is low, an adsorption method is effective as a separation technique. Methods for adsorbing and recovering a rare earth element include a method utilizing a polymer (Patent Literature 3) and a method utilizing a material derived from organisms (Patent Literature 4). Further, commercially available adsorbents, such as strongly acidic cation exchange resins and iminodiacetic acid-based chelate resins, also show adsorption ability for rare earth elements.
However, the above adsorbents have a problem in that they do not have selectivity to rare earth elements. Among base metals, iron is particularly contained in wastes at a high level and is present in various types of waste liquids in combination with rare earth elements. In aqueous acids, iron (III) ions are trivalent ions similar to rare earth element ions. Therefore, iron (III) ions are co-adsorbed with rare earth elements onto an adsorbent, such as a commercially available ion-exchange resin and a chelate resin. Particularly, when the concentration of iron ions is high and the concentration of rare earth elements is low, the above tendency will be remarkable.
On the other hand, in a process of removing iron as an iron hydroxide precipitate, a large amount of alkali is required since the concentration of iron is high; and the iron hydroxide precipitate produced is very poor in filterability and hardly separated from the liquid. Therefore, the process of removing iron is costly and unsuitable for a process of recovering low-concentration rare earth elements.
Further, the process of recovering low-concentration rare earth elements is often performed in a low pH region. However, since an adsorbent utilizing a material derived from a polymer (Patent Literature 3), an adsorbent utilizing a material derived from organisms (Patent Literature 4), an iminodiacetic acid-based chelate resin, and the like develop adsorption ability only in a weakly acidic region, pH adjustment will be required as a pretreatment of the adsorption and recovery process, which increases the cost.
From such a point of view, it is necessary to develop an adsorbent for rare earth element having high selectivity to rare earth element ions in a low-acid solution in the presence of base metal ions in combination with the rare earth element ions.
A solution containing a high concentration of rare earth elements which are relatively easily recovered has been a target of treatment since the price of rare earth elements has been less expensive until now, but from now on, rare earth elements are desired to be recovered from a dilute aqueous solution of rare earth elements which have been discarded so far. That is, in a process of recovering rare earth elements from low-grade natural minerals and wastes, there is desired a method of selectively separating rare earth element ions in a dilute state from an aqueous solution containing metal ions of base metals, such as iron, copper, nickel, and zinc, at a high concentration.
On the other hand, diglycolamic acid is known as an extractant in solvent extraction for selectively separating rare earth element ions and base metal ions (Non Patent Literatures 1 and 2). However, it is well known that when the extractant in solvent extraction is immobilized on a base material, an expected selective separation effect will not be obtained (Patent Literature 5 (paragraph 0009) and Non Patent Literature 3 (p. 90, 2.2)).
An extractant used in a liquid phase system can freely move in the solution and has high intramolecular degree of freedom. Therefore, it has a high diffusion rate and exhibits a high selectivity to a target substance. On the other hand, if a functional group is immobilized, the degree of freedom may be lost, and the selectivity, the adsorption amount, and the rate may be greatly reduced. Particularly in the case of a polydentate system in which high selectivity can be expected, the above tendency is remarkable and a defect.
An impregnation method is a technique of immobilizing an extractant on a base material which improves the above defect. This is a method of impregnating a hydrophobic base material with an organic solvent containing an extractant, in which the original selectivity of the extractant is maintained since the degree of freedom of the extractant is not reduced.
However, since the extractant is held by a physical interaction such as distribution in a solvent, the extractant may be leaked, and the repeated use thereof is difficult. Further, the adsorption rate is small since the interfacial area is small.